Flow sensor

ABSTRACT

Disclosed is a flow sensor for sensing heat transfer with a high-speed response, which is manufactured by applying IC micro-machining technology, and which attains an improved heat transfer efficiency by controlling the flow&#39;s direction between a heating portion and a sensing portion or by utilizing the intrinsic characteristics of the gas flow. 
     A flow sensor having a substrate whereon a heating portion and a sensing portion are formed each in the form of a bridge supported at both ends or at one end in said order in the direction of the flowing gas to be measured, and which is placed, its surface with the elements down, at the upper inside wall of a pipe for gas to be measured. Its output signal is taken out through lead wires.

This is a division of application Ser. No. 08/072,779 filed Jun. 7, 1993U.S. Pat. No. 5,392,647.

BACKGROUND OF THE INVENTION

The present invention relates to a flow sensor and more particularly toa heat transfer detection type flow sensor comprising a heating portionand a heat receiving portion (sensing portion).

Such a thermal-type flow sensor that comprises a substrate is knownwhere on a heating element and a heat-receiving element are disposed atthe upstream side and at the downstream side, respectively, in thedirection of the gas flow to be measured. When gas flows along thesubstrate, it is heated by the heating element at the upstream side andthen the heat is transferred to the heat receiving element at thedownstream side on the substrate. Since the quantity of the transferredheat is proportional to the gas flow, the flow sensor can determine theamount of the gas flow.

However, in the application of the above-mentioned conventional typeflow sensor, the following problems arise.

A divided flow of gas moving along the surface of the substrate, ascompared with a divided flow of gas moving above the surface thereof mayhave decreased flow and a decreased rate of flow consequently has a lowefficiency of heat transfer because of the effect of fluid viscosity andcontact resistance at the boundary's surface.

On the other hand, the divided flow of gas moving above the substratesurface contributes to the flow's measurement by taking heat from theheating element and giving heat to the heat-receiving element, but itcan't sufficiently transfer the heat from the heating element to theheat-receiving element because of the heat's dissipation during itsmovement between the elements. This divided flow has also a lowefficiency in heat transfer.

Furthermore, since the heating element and the heat-receiving elementare resisting the gas flow, the divided flow, apart from the substratesurface, is apt to disappear from the substrate surface. When thesubstrate surface is placed right-side up,there is less heat from theheating element transferred to the heat-receiving element owing to theheat's dissipation by convection (the upward movement of the gas). Thismay cause a decrease in the heat's transfer efficiency.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide ahigh-response, heat-transfer detection type flow sensor which ismanufactured by utilizing micro-machining technology for IC productionand which has an improved efficiency of heat transfer from a heatingelement to a heat receiving(sensing) element by controlling thedirection of the flow of the gas between the elements or by utilizingthe characteristics of the fluid's flow therein.

Another object of the present invention is to provide a flow sensorwhich has an improved efficiency of heat transfer from a heating elementto a heat-receiving element by disposing the heating and theheat-receiving elements under the substrate so as not to allow thedissipation of heat from the heating element by convection (theascending flow of gas).

Another object of the present invention is to provide a flow sensorhaving an improved efficiency of heat transfer with a uniform gas flowrate, wherein an open outlet portion of a gas flow passage is formed toexpand forward and be shortened at both side walls in order to reducethe resistance of the passage against the gas flow along the side wallsof the open-outlet portion at a lower speed than that of the gas flowalong the passage's center line.

Another object of the present invention is to provide a flow sensorhaving an improved efficiency of heat transfer, wherein a heat receivingelement is disposed near an open outlet portion of a gas flow passage toprevent the gas stream above a heat receiving element from going up.

Another object of the present invention is to provide a flow sensorwherein a flow-rectifying plate is provided at the outlet of a gas flowpassage to stabilize the gas stream along the end of a substrate at theoutlet side.

Another object of the present invention is to provide a flow sensorwherein a heating portion and a heat-receiving portion are formed ascoaxial rings to improve the efficiency of the heat's transfer betweenthem.

Another object of the present invention is to provide a flow sensorwherein a flow-rectifying plate is provided for obtaining a uniform gasflow along the heating portion and the heat-receiving portion, andthereby increasing the efficiency of the heat's transfer between them.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view showing the main part of a conventional flowsensor.

FIGS. 2(a), 2(b) and 2(c) are views for explaining a flow sensorembodied in the present invention.

FIG. 3 is a construction view of another embodiment of the presentinvention.

FIG. 4 is a construction view of another embodiment of the presentinvention.

FIGS. 5(a) and 2(b) are views for explaining a part of the process formanufacturing a flow sensor according to the present invention.

FIGS. 6(a) and 6(b) are views for explaining a part of the process formanufacturing a flow sensor according to the present invention.

FIGS. 7(a) and 7(b) are views for explaining a part of the process formanufacturing a flow sensor according to the present invention.

FIGS. 8(a) and 8(b) are views for explaining a part of the process formanufacturing a flow sensor according to the present invention.

FIGS. 9(a) and 9(b) are views for explaining a part of the process formanufacturing a flow sensor according to the present invention.

FIGS. 10(a) and 10(b) are construction views of another embodiment ofthe present invention.

FIG. 11 is a plane view for explaining the function of an outlet portionshown in FIG. 10.

FIG. 12 is a view showing an example of a conventional sensor.

FIGS. 13(a) and 13(b) are construction views of another embodiment ofthe present invention.

FIGS. 14(a) and 14(b) are construction views of another embodiment ofthe present invention.

FIGS. 15(a) and 15(b) are views showing a flow sensor having acantilever sensing portion according to the present invention.

FIGS. 16(a), 16(b) and 16(c) are views for explaining an example of theapplication of a flow sensor.

FIGS. 17(a), 17(b) and 17(c) are construction views of anotherembodiment of the present invention.

FIG. 18 is a construction plane view for explaining a modification ofthe embodiment shown in FIG. 17.

FIG. 19 is a view showing, by way of example, a pattern layout whenusing a silicone (100) substrate.

FIG. 20 is a view for explaining an example of a method for commonlyusing a part of an electrode portion.

FIGS. 21(a) and 21(b) are construction views of another embodiment ofthe present invention.

FIG. 22 is a plane view showing a modification of a throttling plate 60.

FIGS. 23(a) and 23(b) are views showing a modification of theflow-rectifying element shown in FIG. 21.

FIGS. 24(a), 24(b) and 24(c) are construction views of anotherembodiment of the present invention.

FIG. 25 is a construction plane view for explaining a modification ofthe embodiment shown in FIG. 24.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a sectional view showing the essence of the construction of aconventional heat-transfer detection type flow sensor which comprises asubstrate 1, a heating element 2 and a heat-receiving element 3. As iswell known, the heating element 2 and heat-receiving element 3 aredisposed at the upstream and downstream sides, respectively, of thesubstrate 1 in the direction of flow of the gas F to be measured. Whenthe gas F flows in the indicated direction, it transfers heat-generatedby the heating element 2 to the heat receiving element 3. Since thequantity of transferred heat is proportional to the gas's flow, thesensor may determine the rate of flow of the gas. However, a dividedflow D of gas F along the surface of the substrate 1 in comparison witha divided flow U of gas apart from the surface of the substrate 1 mayflow at a lower speed and a rate smaller of flow due to the effect ofthe gas's viscosity and contact resistance at the boundary's surface.Therefore, the efficiency of heat transfer by the divided flow D will below.

On the other hand, the divided flow U of the gas F moving above thesubstrate surface 1 contributes to the flow's measurement by taking heatfrom the heating element 2 and giving heat to the heat-receiving element3, but it can't sufficiently transfer the heat from the heating element2 to the heat receiving element 3 because of the heat's dissipationduring the travel of the gas at a distance there between. This dividedflow U has also a low efficiency of heat transfer.

Furthermore, since the heating element 2 and the heat receiving element3 are resistant to the gas's flow, the divided flow U is apt to go awayfrom the heat receiving element 3. If the substrate 1 is placed "surfaceup" as shown in FIG. 1, the flow of gas heated by the heating element 2ascends by convection and, therefore, does not approach theheat-receiving element 3.

As mentioned above, the conventional flow sensor has a constructionaldrawback that causes low efficiency of heat transfer therein.

In view of the foregoing, the present invention was made to provide ahigh-response, heat-transfer, detection type flow sensor which ismanufactured by utilizing micro-machining technology of IC productionand which has an improved efficiency of heat transfer from a heatingelement to a heat receiving (sensing) element by controlling thedirection of the gas flow between the elements or by utilizing the flowcharacteristics of gas.

Referring to FIGS. 2(a), 2(b) and 2(c), there is shown a flow sensorembodying the present invention. FIG. 2(a) is a sectional view of theessence of the flow sensor, FIG. 2(b) is a sectional view of the flowsensor mounted in gas flow piping and FIG. 2(c) is a sectional viewtaken along line 2C--2C of FIG. 2(b). These drawings show the flowsensor 5 comprising a substrate 1, a heating element (heater) 2 and aheat receiving (sensing) element 3. Numerals 4 and 6 designate,respectively, a lead wire of the flow sensor 5 and a pipe along whichgas flows. As well known, the flow sensor measures the flow F of gasalong the pipe 6 in such a way that heat generated by the heatingelement 2 add diffused in the gas flow is detected by the heat receivingelement 3.

In the embodiment shown of the present invention, the substrate 1 isplace side down (in the direction of gravity) to be located above theheating element 2 and the heat receiving element 3, thereby heat fromthe heating element 2 can't be taken away by convection (upward stream)and the efficiency of heat transfer is greatly improved in comparisonwith a conventional sensor.

In FIGS. 2(b) and 2(c), there is illustrated the flow sensor 5 mountedin a pipe 6 wherein gas flows.

FIG. 3 is a construction view of another embodiment of the presentinvention. A flow sensor comprises a substrate 1, a heating element 2and heat receiving (sensing) elements 3₁ and 3₂ which are disposed intwo (upper and lower) layers in different horizontal planes relative tothe heating element 2 as shown in FIG. 3. Such an arrangement makes itpossible to effectively transfer heat from the heating element 2 to theheat receiving elements even if the heat is diffused by the flow of gasto be measured. The flowing gas F is heated when coming in contact withthe heating element 2 and then is divided into two flows U and D whichmay make contact with the heat-receiving elements 3₁ and 3₂ disposed intwo layers at an adequate distance from each other. Consequently, theheat receiving elements may effectively receive the diffused heat fromthe heating element 2.

FIG. 4 is a construction view of a further embodiment of the presentinvention, wherein a heating element 2 and heat-receiving (sensing)element 3 are arranged at different levels, i.e., the latter beingdisposed higher than the other. This embodiment may have an improvedheat transfer efficiency in comparison with the conventional flowsensor. In the shown case, a divided flow D is apt to go away from thesubstrate 1 owing to contact resistance produced at the boundarythereof. Therefore, it is possible to improve the heat transferefficiency by placing the heat receiving element 3 at the 3unction ofthe two divided flows U and D.

If the heating element 2 and the heat receiving element 3 are of 0.5 to20 microns in thickness and the heat receiving element 3 is separatedfrom the substrate 1 more than from the heating element 2 by a distanceof 1 to 50 microns, it will be very desirable to arrange the elements,for example, as shown in FIG. 4, i.e., the heating element 2 and theheat-receiving element 3 are spaced from each other by 3 microns both invertical and horizontal directions and the heating element 2 is locatedapart from the substrate 1 at a distance of 50 microns.

The method for manufacturing the flow sensor shown in FIG. 3 will bedescribed as follows:

FIG. 5(a) is a plane view and FIG. 5(b) is a sectional view taken online 5B--5B. A silicon (100) substrate 1 is prepared by the knowntechnology of micro-machining semiconductor ICs and a silicon oxide isformed thereon by thermal oxidation. Then a concave shape 10 is formedby photoetching the oxide film or it may be directly formed (withoutforming an oxide film) by plasma-etching through a photoresisting mask.As shown in FIGS. 5(a) and 5(b), the concave 10 is rectangular and mustbe arranged so as not to be parallel on any side to the plane 111 of thesubstrate. It may be of 3 to 50 microns in depth.

FIGS. 6(a) and 6(b) illustrate an insulating layer 11, aheat-resistanting layer 12 and an insulating layer 13 successivelyplaced on the substrate shown in FIGS. 5(a) and 5(b). FIG. 6(a) is aplane view and FIG. 6(b) is a sectional view taken on line 6B--6B ofFIG. 6(a). The heat-resisting layer 12 is formed by using the knownpattern etching technology. The etched pattern consists of a heatingportion 14, a heat receiving (sensing) portion 15, a wiring portion 16and an electrode portion 17.

FIGS. 7(a) and 7(b) are views for explaining a process for forming aspacing layer E shown in FIG. 3 on a substrate's portion correspondingto the concave 10 shown in FIG. 6(a) and 6(b). A spacing layer is formedover the heat receiving (sensing) portion 15 and a spacing pattern 18 isformed thereon by a photoetching process. This pattern 18 is of 3 to 50microns in thickness and must be made of material selectively removableby elution in a subsequent process. An insulating layer 19 for providingthe heat receiving element is further deposited over the pattern 18.

FIGS. 8(a) and 8(b) illustrate the pattern of a heat-resisting layer 20,which is formed on the insulating layer 19 by a like method for forminga similar layer 12 as shown in FIGS. 6(a) and 6(b), and an insulatinglayer 21 formed thereon. FIG. 8(a) is a plane view and FIG. 8(b) is asection taken on line 8B--8B of FIG. 8(a).

FIGS. 9(a) and 9(b) show open electrode portions 22 and an openSi-portion 23 formed by photoetching and plasma etching, from the upperside, insulating layers 21, 19, 13 and 11 shown in FIGS. 8(a) and 8(b).By doing so, there is formed an open area 23 of Si-substrate. A cavity24 is formed by the use of an alkaline anisotropic etching solution.Before or after this, a spacing layer 18 shown in FIGS. 8(a) and 8(b) isremoved by side-etching from the portion wherefrom the insulating layerswere previously removed. As is apparent from FIG. 9(b) showing a sectiontaken on line 9B--9B of FIG. 9(a), there is formed a spacing layer 25corresponding to the spacing layer E shown in FIG. 3. In this case thegas to be measured flows in the direction shown by an arrow F, whichcorresponds to the direction F shown in FIG. 3.

To manufacture a flow sensor of the type shown in FIG. 4, the processshown in FIGS. 6(a) and 6(b) shall be followed by the process shown inFIGS. 9(a) and 9(b). However, in this case the direction of the gas flowis reversed to the flow's direction F shown in FIG. 9(a).

FIGS. 10(a) and 10(b) are views for explaining another embodiment of thepresent invention. FIG. 10(a) is a plane view and FIG. 10(b) is asectional view taken along line 10B--10B of FIG. 10(a). There are showna silicon substrate 30, a heating element 31, a heat receiving (sensing)element 32, a gas passage way and a wall 34 of the passage.

In this embodiment, the heating portion 31 and the heat-receivingportion 32 are bridged over the fluid passage 33 formed in the silicon(100) substrate 30. The heating element 31 and the heat receivingelement 32 are disposed at an edge of the substrate 30 to obtain auniform transfer of heat by utilizing the movement for gas toward thesubstrate. When the fluid flows along the heating element 31 and theheat-receiving element 32, it is divided into two streams U (aboveelements) and D (under elements) as shown in FIG. 10(b). As previouslydescribed in a conventional sensor, the divided flow D is larger thanthe divided flow U.

As previously mentioned, the divided flow of gas U is apt to go awayfrom the heat-receiving element. However, in the case of the heatreceiving element being located near an end-face of portion thesubstrate at a distance of, for example, 5 mm or less, the whole gasflow will be attracted downward along the end face of the substrate,therefore the divided flow U may some close approach to theheat-receiving element. This increases the heat transfer efficiency ofthe sensor.

An open ended portion of the walled gas passage is formed to expandforward and both corner paths of the divided flows L and R are madeshorter than a central path would be for a center divided flow C to makethe wall's resistance to the flow of gas well balanced for uniformlydistributing the flow. This may further improve the heat transferefficiency of the sensor.

FIG. 11 is a plane view of an outlet portion of the passage 33 of gas.Since the open outlet portion of the passage wall 34 is expandingforward, gas flowing along the corner paths L and R may have smallerflow resistance because of the short distance to the end face of thesubstrate 30 in comparison with gas flowing along a center path whichmay have a larger flow resistance due to the large distance to theend-face of the substrate 30. Such a design of the outlet portion mayattain a substantially uniform distribution of the fluxes of gas flowingalong the passage 33.

FIG. 12 is the view of a conventional sensor which has a straight (notexpanded) outlet for a gas passage 33. In this case, even if the uniformflow F of gas enters into the passage, the flow may mostly concentrateat the center line C when passing a heating element 31 and aheat-receiving (sensing) element 32, causing reduced flows of L and Ralong the side wall's of the passage due to the wall resistance.Consequently, only the center portion of the gas low contributes to thetransfer of heat from the heating element 31 to the heat receivingelement 32, resulting in obtaining few detection signals for a wholeoutput. Accordingly, the detection sensitivity is low and noiseseparation is also make difficult.

FIGS. 13(a) and 13(b) are views for explaining another embodiment of thepresent invention. FIG. 13(a) is a plan view and FIG. 13(b) is asectional view taken along line 13B--13B of FIG. 13(a). Componentssimilar in function to those of the embodiment shown in FIGS. 10(a) and10(b) are designated by the same reference numbers. In this embodiment,a substrate 30 is provided near its corner edge with a flow-rectifyingplate 35 projecting thereto or bridged thereon to stabilize the gas flowalong the end-portion of the substrate. This flow rectifying plate 35must be mounted at the end portion of the substrate, otherwise it is ofno use.

FIGS. 14(a) and 14(b) are views for explaining another embodiment of thepresent invention. FIG. 14(a) is a plane view and FIG. 14(b) is asectional view taken along line 14B--14B of FIG. 14(a). Componentssimilar in function to those of the embodiment shown in FIGS. 13(a) and13(b) are designated by the same reference numbers. In this embodiment,a substrate 30 is a silicon wafer <Si(100)> whereon a passage 33 isformed at high accuracy of its width between side walls 34 in thefollowing manner:

A saw-tooth profile of each side wall is precisely patterned on aninsulating layer, keeping each front face (of the profile) parallel tothe basic plane (111) and each back faces at a right angle thereto, thenthe silicon substrate is etched according to the pattern by ananisotropic etching method to form the message 33 thereon.

A flow rectifying plate 35 in the form of a bridge is formed at the sameinsulating layer where a heating element 31 and a heat-receiving(sensing) element 32 are formed so as to align it with the elements atabout the same plane. This flow rectifying plate 35 shall be located atthe edge 30' of the substrate 30 or be projected beyond said edge. Theplate 35 across the flow of gas shall be i.e. wide not less than that ofthe heating element 31 and the heat-receiving element 32 to exert theintended effect to rectify the gas flow. If there is no rectifyingplate, the gas, having passed over the edge of the substrate, runsaround the sides of the substrate to form thereat vortexes that may growto unstable vortex currents as the flow increases and effects heattransferring from the heating element to the heat-receiving element. Theaccurate measurement of the flow therefore can not be performed.

FIGS. 15(a) and 15(b) are views fop explaining another embodiment of thepresent invention. FIG. 15(a) is a plane view and FIG. 15(b) is asectional view taken along line 15B--15B of FIG. 15(a). In thisembodiment, a heating element 31 and a heat receiving (sensing) element32 are formed each in the form of a cantilevered bridge across thepassage 33 on the substrate 30. The heat-receiving element 32 isarranged at a higher level than the heating element 31, for example, asshown in FIG. 4 if viewed from the direction Y--Y of FIG. 15(a).

FIGS. 16(a), 16(b) and 16(c) are views for explaining an example of howto use a flow sensor thus manufactured. FIG. 16(a) shows, inperspective, the sensor mounted on a wedge-shaped stand 41, FIG. 16(b)is a sectional view of a pipe 42 whereon the stand 41 with the sensorsecured thereto is mounted, and FIG. 16(c) is a sectional view taken online 16C--16C of FIG. 16(b). In FIG. 16(a), the whole structure of theflow sensor, integrally secured to the stand, is illustrated with thereference numeral 40. A wedge-shaped stand is desired in order to havean angle α of 1°˜40° at the inlet side and an angle β of 60°˜179° at theoutlet side of the pipe. Mounting the sensor on the stand 41 eliminatesthe possibility of vortexes occurring when gas is passing the flowsensor transferring the heat from the heating element 31 to theheat-receiving (sensing) element 32.

FIGS. 17(a), 17(b) and 17(c) are views for explaining another embodimentof the present invention. FIG. 17(a) is a plane view, FIG. 17(b) is asectional view taken on line 17B--17B of FIG. 17(a). FIG. 17(c) is aview of a modification of the embodiment shown in FIG. 17(b).

In these drawings, numeral 50 designates a substrate made of a materialsuch as Cu, Ni, Cr, Si, stainless steel, copal (polystyrene), polyimidand so on, numerals 51 and 52 designate insulating layers made of aheat-resisting material such as MgO, SiO₂, Ta₂ O₅, Si₃ N₄, Al₂ O₃ and soon, and numerals 53 and 54 designate a heating layer and aheat-receiving (sensing) layer, respectively, made of material forresisting heating such as Pt, NaCr, W, SiC, Kanthal and so on. In thisembodiment, the heating layer 53 and the heat-receiving layer 54 arerings arranged concentrically with each other and at a gap of 55therebetween.

They are both projecting over a through hole 56 (FIG. 17(b)) or a cavity57 (FIG. 17(c)) formed on the substrate.

The embodiment is intended to improve the efficiency of heat transfer bydiffusing heat from the heating element (inner ring) in a radial,peripheral direction, and transferring it to the heat-receiving element(outer ring). When the flow sensor of this embodiment is located in apipe in such a way that gas F flows in a perpendicular direction to thesensor's surface or passes through a hole 61 of a throttling plate 60and enters at approximately the center of concentric rings, the gas flowis evenly distributed toward the outside of the rings to evenly transferheat from the heating element 53 to the entire ring's surface of theheat receiving element 54. Consequently, the sensor can effectivelysense the gas's flow. While FIG. 17 shows the single-ring type elements53 and 54, they may be of a multi-ringed type or be formed into arectangle or in a curved shape. The reverse direction of the flow of gasis also possible. A hole 58 provided at the neck portion of thering-shaped heating element 53 makes it easier to etch the neck portionby allowing an etching solution therethrough.

FIG. 18 is a plane view for explaining a modification of the embodimentshown in FIGS. 17(a), 17(b) and 17(c). In this modified embodiment, aheating layer 53 and a heat-receiving layer 54 are formed each in a filmof a zigzag pattern that is more effective for generating heat and toabsorb it, and further it improves the sensitivity and resolution of theflow's sensor. As shown in FIG. 18, the provision of notches 55' in acavity 55 can increase the heat transferring efficiency of the sensor bypreventing an excessive increase of temperature.

FIG. 19 shows an example of an arrangement of patterns on a silicon(100) substrate 50. As shown in FIG. 19, a lead pattern from aconcentric circle is mounted at 45° to the plane face Si(111). Athrough-hole and a cavity is formed by etching the substrate Si througha hole 58 in order not to leave a residue of plane face (111) around thehole 58. Consequently, the lead pattern is separated from the substratewhich may prevent heat loss when the heat is being transferred to thesubstrate. This improves the heat's distribution, heat transferefficiency and durability of the sensor. While there are shown separatedelectrode leads, it is also possible to form a commonly usableshort-circuit between one end of the heat-receiving element and one endof the heating element. This can be made by connecting wiring patternson the substrate. For example, portions A and B of the wiring patternsshown in FIG. 19 are connected with each other as shown in FIG. 20.Accordingly, the quantity of leading electrodes may be reduced to threepieces as shown in FIG. 20.

FIGS. 21(a) and 21(b) are views for explaining another embodiment of thepresent invention. FIG. 21(a) is a plane view and FIG. 21(b) is asectional view taken on line 21B--21B of FIG. 21(a). Components similarin function to those of the embodiment shown in FIGS. 17(a), 17(b) and17(c) are denoted by the same reference characters used therein. In thisembodiment, a number of flow rectifying plates 62 are arranged radiallyaround a throttling hole 81 of a throttling plate 60 to reduce deviationof heat transfer. These flow rectifying plates can be disposed on thesensor substrate.

As shown in FIGS. 21(a) and 21(b), it is also possible to achievefurther reduction of the flow deviation by providing a flow rectifyingcone 59 opposite the center of the throttling hole 61.

FIG. 22 is a plane view for explaining an alternate embodiment of athrottling plate 60, which has a throttling hole 61 having notches orprojections 61' radially formed thereat for uniformly adjusting thegas's flow.

FIGS. 23(a) and 23(b) show an example of a modification of theflow-rectifying cone 59 shown in FIGS. 21(a) and 21(b). FIG. 23(a) is atop plane view of the flow-rectifying plate 60 and FIG. 23(b) is asectional side view of said plate. The provision of the rotary vane-typeflow rectifying cone 59 assures obtaining a further uniform distributionof the flow rate.

FIGS. 24(a), 24(b) and 24(c) are views for explaining another embodimentof the present invention. FIG. 24(a) is a plane view, FIG. 24(b) is asectional view taken on the plane of the line 24B--24B of FIG. 24(a) andFIG. 24(c) is a sectional view taken on the plane of the line 24C--24Cof FIG. 24(a) wherein a flow rectifying plate is omitted forclarification. Components similar in function to those of the embodimentshown in FIGS. 17(a), 17(b) and 17(c) are denoted by the same referencecharacters used therein.

This embodiment is characterized in that a heating portion 53 is of adouble-circular type; a part of the electrodes of the heating portionand a heat-receiving (sensing) portion is used as a common electrode; apart (L) of a sensor substrate is in a circular form; and the heatinglayer 53 and the heat-receiving (sensing) layer 54 are formed atdifferent levels. Especially, the application of the substrate beingpartially circular is effective to adjust the gas flow more uniformlyand the employment of a throttling plate 60, having sizes substantiallycorresponding to those of the substrate (some difference may be allowed), forces the gas to pass through its throttle hole 61 and then to evenlyflow along the substrate by the sucking-out effect of the gas flow alongits outside.

FIG. 25 shows a modified embodiment of the flow sensor shown in FIGS.24(a), 24(b) and 24(c). The modified sensor is featured by that of atapered portion 59, formed by etching at the center of a substrate 56,serves as a flow rectifying element capable of effectively diffusing thegas flow. The circumferential portion 51' of the substrate, which isformed by etching to be thinner than the center portion, is effective toadjust the gas flow smoothly. It is convenient that the flow-rectifyingportion can be formed integrally with the substrate.

As apparent from the foregoing, the present invention provides:

a flow sensor which has an improved efficiency of heat transfer from aheating element to a heat receiving (sensing) element by placing theheating element and the heat-receiving element under the substrate so asnot to allow the dissipation of heat from the heating element throughconvection (the ascending flow of gas);

a flow sensor, wherein a heat receiving (sensing) element is disposed ata level higher than the heating element or two heat receiving elementsin two stages at different levels, respectively, higher and lower than aheating element to effectively transfer heat from the heating element tothe heat receiving element or elements, even if the heat from theheating element ascends or is diffused by the gas flow to be measured;

a flow sensor having an improved efficiency of heat transfer with auniform gas flow rate, wherein an open outlet portion of a gas passageis formed to expand forward and be shorter at both sides than its centerline portion to reduce the passage's resistance against gas flow alongthe side walls of the open outlet portion lower than that of gas flowalong the passage's center line;

a flow sensor having an improved efficiency of heat transfer, wherein aheat-receiving (sensing) element is disposed near an open outlet portionof a gas passage to prevent gas from flowing above the heat-receivingelement i,e, from flowing upward;

a flow sensor wherein a flow-rectifying plate is provided at the outletof the gas passage in order to stabilize the flow of gas along theoutlet end of a substrate;

a flow sensor, wherein a heating portion and a heat-receiving portionare formed as coaxial rings to improve the efficiency of heat transferthere-between; and

a flow sensor wherein a flow-rectifying plate is provided to obtain auniform flow of gas along a heating portion and a heat receiving(sensing) portion, thereby increasing the efficiency of heat transferthere-between.

I claim:
 1. A flow sensor comprising a substrate having a through holeor a cavity therein, a heating layer disposed to form substantially aring and a heat-sensing layer disposed around and coaxially with theheating ring layer at a certain even distance therefrom, characterizedin that the heating layer and the heat sensing layer are partiallyprojecting above the through hole or the cavity.
 2. A flow sensoraccording to claim 1, characterized in that a throttling plate having athrottling hole therein is disposed on the substrate in such a way thatflowing gas to be measured passes through the throttling hole of thethrottling plate and enters into the center portion of the ring-shapedheating layer.
 3. A flow sensor according to claim 1, characterized inthat the heating layer and/or the heat sensing layer are/is or is madein a zigzag form.
 4. A flow sensor according to claim 1, characterizedin that the substrate is a silicon (100 substrate whereon lead patternsfrom the heating layer and the heat sensing layer are formed at an angleof 45° to a silicon (111) plane of the substrate.
 5. A flow sensoraccording to claim 2, characterized in that a plurality of flowrectifying plates are radially disposed underneath and around thethrottling hole of the throttling plate to oppose the heating layer andthe heat sensing layer.
 6. A flow sensor according to claim 2,characterized in that a flow rectifying cone is disposed on a plane ofthe substrate in such an away as to substantially oppose the center ofthe throttling hole of the throttling plate disposed on the oppositeplane of the substrate.
 7. A flow sensor according to claim 2,characterized in that the throttling hole of the throttling plate hasnotches or projects radially arranged around the circumference thereof.8. A flow sensor according to claim 2, characterized in that a rotaryvane-type unit for rectifying the flow's diffusion is disposed at such aposition on a plane of the substrate, where it may substantially opposethe center of the throttling hole of the throttling plate disposed onthe opposite plane of the substrate.